1. Field of the Invention
The present invention relates to a developer for electrophotographic copying machines, and more particularly to a carrier of two-component developer for electrophotographic copying machines.
2. Description of the Prior Art
Iron powder, ferrite powder, or the like have been used as carriers of two-component developers for electrophotographic copying machines. These carriers usually have a specific resistance of about 10.sup.6 .OMEGA. cm, representing the conductive characteristic, and of about 10.sup.12 .OMEGA. cm, representing the dielectric characteristic.
When using a conductive carrier having a specific resistance of about 10.sup.6 .OMEGA. cm, an injection of an electric charge from a developing roll is effected and the actual developing electric field is enlarged, so that a solid black portion can be developed with high intensity. The term "solid black portion" means a solid black area of an original document to be copied. In these instances, however, there has sometimes occurred undesirable white lines within the solid black portion causing poor reproduction of a thin line.
On the other hand, when using a dielectric carrier having a specific resistance of about 10.sup.12 .OMEGA. cm, the relationship between the developing electric field and the spatial frequency, i.e., number of lines/mm, is as shown in FIG. 1. The maximum value of the developing electric field is within the density region of 1.0 to 10 lines/mm, which means that the reproducibility of a thin line is excellent. The electric field for development becomes too weak, however, because the injection of the electric charge from the developing roll is not effected in the solid black portion and an electric charge with a polarity opposite to the polarity of the toner is retained on the carrier on the surface of the dielectric developer layer after development. A dielectric carrier, therefore, has the disadvantage of a so-called edge effect, whereby the toner density at the central portion of the solid black portion is reduced in comparison with that at the edge portion.